1. Field of Invention
The present invention relates to a method of planarization. More particularly, the present invention relates to a method including the step of forming a spin-on-glass (SOG) layer over a semiconductor wafer prior to performing a chemical-mechanical polishing process, so that a smooth polished wafer surface is obtained.
2. Description of Related Art
In the fabrication of VLSI or ULSI circuits, all the devices, metallic interconnects or isolating trench structures on a semiconductor wafer must be carefully laid out so that the level of device integration and operating speed are optimized. However, these devices and structures are likely to be distributed non-uniformly above the semiconductor wafer. Consequently, a portion of the wafer surface will be densely packed while some other areas may have very few devices or isolating structures. Furthermore, all these devices and structures have different thicknesses. Hence, the wafer is likely to have a highly undulating cross-sectional profile. To facilitate the next processing process, the semiconductor wafer is usually planarized.
At present, the most commonly used method of planarizing a semiconductor wafer is the chemical-mechanical polishing (CMP) method. In a CMP process, chemical agents in the form of slurry are delivered to a rotating polishing table with a polishing pad on top. With the back of a wafer grasped by a handle, a global polishing of the wafer is carried out by pressing the front surface against the polishing pad though the handle. Since the polishing pad is made from a soft, flannel-like material, the pad is able to follow the varying contour of the wafer surface.
Because devices and structures are not evenly distributed on the surface of the semiconductor wafer, some areas are densely packed while other areas are loosely packed. Therefore, when an insulation layer is deposited over the semiconductor wafer, the insulating material will accumulate to a higher level in the densely packed areas than in the loosely packed areas. Subsequently, if a chemical-mechanical polishing process is carried out to planarize the wafer surface, recess cavities can easily form in the loosely packed device areas. Hence, a completely planarized semiconductor wafer surface is difficult to obtain.
FIGS. 1A through 1C are schematic, cross-sectional views showing the progression of steps according to a conventional method of planarizing an insulation layer above a semiconductor wafer.
First, as shown in FIG. 1A, a semiconductor wafer 100 having a densely packed device region 108 and a loosely packed device region 110 thereon is provided. Next, an insulation layer 102 is formed over the substrate 100 in both regions 108 and 110. The insulation layer can be formed by performing a chemical vapor deposition (CVD) process. The insulating material of the insulation layer 102 accumulates to a higher level in the densely packed device region 108 than in the loosely packed device region 110. Hence, the upper surface of the insulation layer 102 has a highly undulating profile.
Next, as shown in FIG. 1B, a chemical-mechanical polishing (CMP) process is carried out to planarize the semiconductor wafer 100. Due to the relative softness of the polishing pad 104, the polishing pad 104 is able to contact the upper surface of the insulation layer 102 in the loosely packed region 110 more closely than the densely packed region 108.
Thereafter, as shown in FIG. 1C, a recess cavity 106 is formed in the loosely packed device region 110 so that a not-so-planar insulating layer 102 results. Consequently, when material is deposited over the wafer, thickness tends to vary across the surface. Additionally, when the wafer is subsequently etched, depth of etching will tends to vary across the surface, too. Hence, more defects accumulate on the wafer and failure rate of the devices within the wafer is higher.
In light of the foregoing, there is a need to provide an improve method for planarizing a semiconductor wafer.